This invention relates to an adaptive control for an induction motor drive system and more particularly to a microcomputer-based real time pole identification and adjustment scheme for an induction motor drive system.
DC machine drives are being replaced by induction motor drives in high performance industrial applications, such as servo drives in robotics and numerically controlled machines to take advantage of rugged, low maintenance induction motors. In such applications, the drive system is not only required to be stable, but should have a fast and predetermined transient response. In complex control systems, a mathematical model of the system showing the relationship between the system variables is typically used to analyze the system. Since the system under consideration is dynamic in nature, the descriptive model contains differential equations. If the equations can be linearized, then Laplace transforms can be utilized to simplify their solution.
A fast, predetermined transient response in a system requires that the poles of the drive system be assigned unique locations irrespective of operating point. The poles of the drive system are determined from the transfer function, defined as the ratio of the Laplace transform of an output variable to the Laplace transform of an input variable. Poles are defined as critical complex frequencies where the transfer function becomes infinite. The form of the transient response of the closed loop system depends on the pole locations of the transfer function of the closed loop system drive.
It is well known that an induction motor drive constitutes a nonlinear multivariable system and, therefore, the transient response dictated by the poles varies at each steady state operating point. A conventional control system is designed with fixed control parameters so that for the worst pole locations the system is stable and its transient response is satisfactory. However, the transient response of such a conventional control system disadvantageously differs at each operating point.
Previous attempts have been made to represent a drive system by a simplified linear model and to place the poles by state variable feedback through a fixed topology feedback gain matrix. See, for example, B. K. Bose, "Adjustable Speed AC Drives--A Technology Status Review", Proceedings of the IEEE, Vol. 70, No. 2, February 1982, pp. 116-135, herein incorporated by reference, where it is pointed out that the gain matrix of the controller can locate all the poles of the system if state variable feedback is used.